1. Field of the Invention
This invention relates in general to a process for the production of discontinuous metal matrix composites and, more particularly, to a process for production of Ingot-metallurgy discontinuous compounds of B.sub.4 C/Al-Li,B.sub.4 C/Al, SiC/Al, SiC/Mg, etc.
2. Description of Related Art
Discontinuous matrix metal composites (MMCs) such as SiC whisker and particulate composites are prepared via powder metallurgy (PM). Typically the powders of the composite components such as aluminum and the SiC in desired concentration are mixed intimately. Numerous techniques for mixing are used. Blending in a V blender, in a paint shaker, in a polymeric vehicle, liquid medium (e.g. alcohol or benzene), etc. can be used. Almost invariably, however, the mixture is poor, i.e., the distribution of the two components is not even. This is simply due to the differences in the specific gravity, the particle size and shape of the matrix metal powder and the SiC powder, the particle size distribution and the volume of each component. For example, SiC density is 3.12 g/cc as compared to 2.7 g/cc for Al. The volume fraction of SiC used in a typical composite is 0.20, the rest being Al or its alloy. The average particle diameter (APD) of Al powder is 15 microns (or -325 mesh). The SiC particle APD is 5 microns. The shape or the morphology of Al and SiC are necessarily different because each is prepared by a different method; Al powder is manufactured via atomization while SiC is obtained from the reaction of silica and carbon followed by comminution (as in ball milling) to break down the large chunks.
After a mixture of the two components is obtained, it is necessary to compact them via cold and hot pressing in a die. Since Al powder is always surrounded by a thin layer of its oxide, namely Al.sub.2 O.sub.3, the compacting must be done under pressure to break the oxide film and generate new fresh Al surface to bond to itself and the SiC. Often, the hot pressing must be done in vacuum to get good compacting and to remove any water of hydration that may exist on the Al powder surfaces. If the outgassing of the water of hydration is incomplete, there is a strong probability of forming hydrogen when the composite billet is heated for secondary forming such as extrusion, rolling, or welding, as the case may be.
While the example cited above is for SiC/Al composite, it is also true of other MMCs such as SiC/Mg and B.sub.4 C/Mg composites produced via powder metallurgy. In fact, the affinity of finely divided Mg towards O.sub.2 is even greater than that of Al. This is due to the differences in the nature of the oxides that form on Al and Mg. Aluminum oxide Al.sub.2 O.sub.3 is adherent to the underlying Al, but the magnesium oxide (MgO) on Mg is porous. Thus, the oxidation protection provided by Al.sub.2 O.sub.3 is far superior to that of the MgO. During compaction of the composite, further severe oxidation occurs and the MgO content rises to a significant extent.
Based on the above, it is clear that an ingot metallurgy route to preparation of MMCs is needed. There are, of course, many attempts made in the past to utilize ingot metallurgy to produce discontinuous MMCs. Only one successful process is known, but that process is limited to a casting alloy of Al, A356, which contains 6% Si. This process apparently involves a treatment of SiC particles, about microns in diameter, so that the latter is easily introduced into molten Al alloy A356, Most of the procedure is proprietary and no more than 15 volume percent of SiC can be incorporated into the melt. More desirable matrix alloys such as 6061, 7075, and 2024 are not available. To prepare composites from these alloys, powder metallurgy must be used with attendant problems of oxide contamination, distribution, outgassing, etc., as described above.
U.S. Pat. No. 4,743,511 discloses a cermet particle comprising a continuous ceramic phase and a discontinuous metal phase. U.S. Pat. No. 4,022,584 discloses a composite material of aluminum oxide and refractory transition metal diborides with addition of magnesium oxide. U.S. Pat. No. 4,224,380 discloses bonding a mass of abrasive particles to form an abrasive body. A metallic phase can contain manganese, and alloys of aluminum U.S. Pat. No. 3,725,015 discloses forming a refractory shape by mixing particulate refractory material with a carbon-containing substance, i.e., boron carbide powder mixed with furfuryl alcohol to a desired shape, treating to convert the carbon-containing substance to carbon, and impregnating the structure with a molten metal (silicon, aluminum and boron alloy).
U.S. Pat. No. 3,492,114 discloses metal constituents which are to be alloyed in an alloy or steel melt which are added in the form of an oxide to the lining of the treatment vessel containing the melt so that upon addition of lithium to the melt the lithium replaces the metal constituents of the oxide to free the melt constituents for alloying with the melt. U.S. Pat. No. 4,548,774 discloses a process in which a matrix material is introduced into a fibrous base comprising a spongelike cake form of SiC whiskers. Matrix metals include Mg, Al, Mn, etc. U.S. Pat. No. 3,421,862 discloses a high-strength whisker composite article comprising an alloy matrix which is wetted to single crystal, and non-metallic whiskers (silicon carbide and boron carbide). The matrix can be aluminum or magnesium. A small amount of lithium can be included. The patent discloses intimately mixing powders of the pre-alloy and whiskers and then heating to produce the desired product. Hot pressing, sintering and cold pressing are employed. U.S. Pat. No. 3,999,954 discloses a hard metal body of a bonding metal of iron, cobalt and nickel and a hard metal refractory carbide such as titanium.
U.S. Pat. No. 4,012,204 discloses a composite of polycrystalline alumina fibers in a matrix of an aluminum alloy containing 0.5-5.5% of lithium. U.S. Pat. No. 4,547,435 discloses a composite of a matrix metal (magnesium) and inorganic fibers (silicon carbide fibers, and boron carbide fibers). This patent discloses deterioration of the fibers in contact with the melted metal. There is also mention of using lithium in a small amount in an aluminum matrix. U.S. Pat. No. 4,053,011 discloses a composite of alumina fibers in an aluminum alloy containing small amounts of lithium. Silica coatings on the fibers promote wetting by aluminum-lithium alloys. U.S. Pat. No. 3,890,690 discloses a metal matrix of Al or magnesium and reinforcing members of silicon carbide and boron carbide.